Osama bin Laden, mastermind of the Sept. 11, 2001, terrorist attacks, has been killed by U.S. forces in what is being described as a surgical strike at a compound in northern Pakistan, ending one of the longest and costliest manhunts in history.

President Obama announced the news late Sunday at the White House, calling the death of bin Laden "the most significant achievement to date" in the war against al-Qaida — a battle that has led the U.S. into protracted and bloody conflicts in both Afghanistan and Iraq.

"Justice has been done," Obama said.

The news released a decade's worth of emotion as Americans, cheering, waving flags and singing the national anthem, streamed to the site of the World Trade Center in New York City, the gates of the White House and across the nation.

Obama said U.S. intelligence tracked the terrorist leader to a redoubt near the Pakistani town of Abbottabad. His movements were monitored for months until a small team of U.S. operatives moved on the compound early Monday local time.

"After a firefight, they killed Osama bin Laden and took custody of his body," the president said, warning that the U.S. must remain vigilant because al-Qaida will "continue to pursue attacks against us."

American officials told NPR that the 40-minute operation was spearheaded by an elite group of U.S. Navy special forces called SEAL Team 6. The officials, who asked not to be named, said SEAL Team 6 took over the search for bin Laden eight years ago when the Army's elite Delta Force soldiers were deployed to Iraq. CIA operatives were also on the ground to help with the operation.

Bin Laden was shot in the head during the assault, the officials said. NPR confirmed that his remains were buried at sea in accordance with Islamic tradition that calls for a speedy interment of the body. Several other adults also were killed, including a son of bin Laden and a woman who reportedly was used as a human shield.

Obama said there were no American casualties in the assault.

Even before the Sept. 11 attacks that killed nearly 3,000 people and became a defining moment in U.S. history, American officials had bin Laden in their sights. But he managed to elude capture, moving to Sudan and later to Afghanistan, where he was sheltered by the Taliban even after the regime was toppled by a U.S. invasion. Bin Laden is thought to have continued a secret nomadic existence, moving in and around the mountainous border region that straddles Afghanistan and Pakistan.

Obama said that almost two years ago, he formally ordered the CIA to make finding bin Laden a top priority. The administration said years of intelligence gathering began to pay off in August, when authorities discovered a heavily fortified compound outside Islamabad that appeared to be custom-built for harboring someone as notorious — and resourceful — as bin Laden.

"I met repeatedly with my national security team as we developed more information about the possibility that we had located bin Laden hiding within a compound deep inside of Pakistan," he said. "Finally, last week, I determined that we had enough intelligence to take action, and authorized an operation to get Osama bin Laden and bring him to justice."

The order was given Friday morning, U.S. officials said, shortly before the president left to tour tornado-raked areas in Alabama. Officials said the final operation — which was under the direction of CIA head Leon Panetta — was so secret that no foreign officials were informed and only a small circle in Washington was aware.